Optically active rhodium complexes of 3,4-bis(diarylphosphino)pyrrolidines and their use for the preparation of phosphinothricin by asymmetric hydrogenation

ABSTRACT

The invention relates to rhodium complexes of the formula 
     
         [A--O--(CH.sub.2 CH.sub.2 O).sub.n --A].sup.2+ 2 X.sup.-   Ia 
    
     and 
     
         [A--O--(CH.sub.2 CH.sub.2 O).sub.m --CH.sub.3 ].sup.+ X.sup.-Ib 
    
     in which 
     n=5-1,000, 
     m=5-250, 
     X -  denotes a tetrafluoroborate, hexafluorophosphate or per-chlorate anion, 
     A denotes a radical of the formula ##STR1## (en) 2  denotes two molecules of a monoolefin or one molecule of a diolefin 
     Ar denotes phenyl or phenyl which is substituted by one or two alkyl groups having 1 or 2 carbon atoms and 
     R 1  denotes an arylene or alkylene bridge, and their use for the preparation of L-phosphinothricin and its derivatives by enantioselective catalytic hydrogenation of 2,3-dehydrophosphinothricin (derivatives).

The invention relates to novel optically active rhodium complexes of thegeneral formula

    [A--O--(CH.sub.2 CH.sub.2 O).sub.n --A].sup.2+ 2 X.sup.-   Ia

and

    [A--O--(CH.sub.2 CH.sub.2 O).sub.m --CH.sub.3 ].sup.+ X.sup.-Ib

in which

n=5-1,000,

m=5-250,

X⁻ denotes a tetrafluoroborate, hexafluorophosphate or perchlorateanion,

A denotes a radical of the formula ##STR2## (en)₂ denotes two moleculesof a monoolefin or one molecule of a diolefin

Ar denotes phenyl or phenyl which is substituted by one or two alkylgroups having 1 or 2 carbon atoms and

R¹ denotes an arylene or alkylene bridge.

The invention furthermore relates to the use of the catalysts for thepreparation of L-homoalanin-4-yl-(methyl)-phosphinic acid(L-phosphinothricin, L-Ptc) and their derivatives by enantioselectivecatalytic hydrogenation of 2,3-dehydro-phosphinothricin (derivatives).

Phosphinothricin (Ptc) of the formula ##STR3## and its esters and saltsare known as active herbicides from DE-PS No. 2,717,440 (U.S. Pat. No.4,168,963). As a result of the asymmetric carbon atom (identified by anasterisk), it occurs in two enantiomeric forms, the L-form of which isthe carrier of the physiological activity (DE-OS No. 2,856,260 andGB-A-2,011,416). It is already known that L-Ptc and its derivatives canbe prepared by enantioselective (asymmetric) hydrogenation of2,3-dehydro-phosphinothricin derivatives of the formula ##STR4## inwhich R denotes hydroxyl, (C₁ -C₆)aloxy, Ala-Ala(OH) or Ala-Leu(OH)

R₁ denotes H or (C₁ -C₆)alkyl and

R₂ denotes an acyl, alkoxycarbonyl or aryloxycarbonyl radical, by meansof rhodium catalysts and subsequent splitting off of the acyl orcarboxylic ester radical (DE-OS No. 3,609,818, ZA No. 87/2,058).However, the profitability of the process when carried out on anindustrial scale depends on a number of conditions which are met onlyinadequately by the catalysts described in DE-OS No. 3,609,818.

It is known from Chem. Ber. 119, 3326 (1986) that the solubility of theRh catalyst in the chosen solvent is a precondition of thehydrogenation, and that the conversion rate (mol of substrate per mol ofcatalyst per unit time) depends on a catalyst-specific minimumconcentration of catalyst. If the concentration falls below this minimumconcentration, the conversion rate drops rapidly.

Because of the high costs of Rh catalysts, the profitability of thehydrogenation process also depends on the ratio of the amounts ofsubstrate : catalyst. It goes without saying that the process can becarried out more cheaply the less catalyst is required, that is to saythe higher the concentration of the substrate to be hydrogenated, basedon the minimum concentration needed for the catalyst. This ratio isinfluenced decisively by the solvent used.

Since 2,3-dehydro-phosphinothricin and its derivatives are particularlyreadily soluble in water, it is obvious to choose water as the mediumfor an economic hydrogenation process. However, the Rh catalysts knownfrom DE-OS No. 3,609,818 are not sufficiently soluble in water, so thatthe minimum concentration required is not reached. On the other hand, inorganic solvents such as methanol, the substrate(2,3-dehydro-phosphinothricin) does not have the optimum solubility, sothat the ratio of the amounts of substrate/catalyst is unfavorable.

There was thus the object of discovering rhodium catalysts of goodwater-solubility which enable 2,3-dehydrophosphinothricin, which islikewise readily soluble in water, to be hydrogenated using a highratio, that is to say a ratio which is suitable for an economic process,of the amounts of substrate:catalyst. This object is achieved by thecatalysts of the formula I according to the invention.

They are obtained by reacting a polyglycol of the formula HO--(CH₂ CH₂O)_(n) --H or a polyglycol monomethyl ether of the formula HO--(CH₂ CH₂O)_(m) --CH₃ with a diisocyanate of the formula R₁ (N═C═O)₂,derivatizing the resulting addition product with a3,4-bis(diarylphosphino)pyrrolidine of the formula ##STR5## and reactingthe reaction product with a rhodium complex of the formula

    [Rh(en).sub.2 Y].sub.2                                     VI

(Y=Cl, Br or I) and an alkali metal salt or silver salt oftetrafluoroboric acid, hexafluorophosphoric acid or perchloric acid.

The individual components can be employed in stoichiometric amounts inthe process described above for the preparation of the catalysts of thegeneral formulae Ia and Ib according to the invention. Advantageously,however, in the reaction sequence shown, the particular component fornew addition is employed in an amount less than the stoichiometricamount, preferably of 5-20 mol %. This should mean that all the newcomponents react as quantatively as possible with the polyglycol orpolyglycol monomethyl ether which is becoming progressively morefunctionalized. It is furthermore advantageous here to allow isocyanategroups still present in the reaction mixture to react by addition of analcohol, such as ethanol or methanol, before addition of the rhodiumcomponent. The catalysts of the formulae Ia and Ib according to theinvention are of course not obtained as pure substances in this manner,but as a mixture with compounds which are derived from the generalformulae Ia and Ib such that, in the case of compounds of the formulaeIa and Ib, a radical of the general formula II, and in the case of thecompounds of the formula Ia, also two radicals of the general formulaII, are not built up completely.

In order to allow direct comparison of the hydrogenation activity of thecatalysts thus obtained with low molecular weight uniform catalystswhich are known from the literature, they are advantageouslycharacterized by an average molecular weight based on rhodium, it beingpossible for this weight to be easily calculated from the amount ofrhodium complex of the formula VI added during the synthesis.

In the starting substances and end products, the individual radicals andsubstituents have the following meaning.

"en" is a straight-chain, branched or cyclic mono- or diolefin, forexample ethylene, 2-butene, butadiene, isoprene, cyclohexene,cyclooctene, 1,5-cyclooctadiene or norbornadiene.

The nature of the radical R¹ results from the structure of thediisocyanate employed, for which commercially available di-isocyanatesare suitable. Examples of these are hexamethylene diisocyanate,isophorone diisocyanate, diphenylmethane diisocyanate or 2,4-toluylenediisocyanate. Diisocyanates with a graduated reactivity of theindividual isocyanate groups, such as, for example, 2,4-toluylenediisocyanate, are particularly suitable.

Suitable polyethylene glycols are those having 5-1,000 EO units(ethyleneoxy units), corresponding to an average molecular weight ofabout 250 to 45,000. n of 50-250, corresponding to a molecular weight ofabout 2,000 to about 11,000, are preferred. Suitable polyethylene glycolmonomethyl ethers contain 5-250 EO units, corresponding to molecularweights of about 250 to 11,000; m of 50 to 150=molecular weight˜2,000 to6,000 are preferred.

The optically active 3,4-bis(diarylphosphino)pyrrolidines of the generalformula V in which "Ar" has the meaning already given are described inChem. Ber. 113, 3426 (1986) or EP-OS No. 151,282 (U.S. Pat. No.4,634,775) or can be prepared by the process described therein byreaction of optically active 3,4-dimethanesulfonylpyrrolidinium bromideor acetate with an alkali metal diarylphosphide. If (+)-tartaric acid isused as the starting substance in these multi-stage syntheses, the(R,R)-3,4-bis(diarylphosphino)pyrrolidines are finally obtained.

The rhodium complexes of the formula VI are likewise known from theliterature (for example JACS 93, 3059 (1971)), or they can be preparedby processes analogous to those described therein; some of them are evencommercially available.

The rhodium complexes of the formula I in which (en)₂ denotes a1,5-cyclooctadiene molecule and X⁻ denotes a tetrafluoroborate anion canalso be prepared in a particularly simple manner by reacting thederivatized 3,4-bis(diarylphosphino)pyrrolidines directly with a rhodiumcomplex of the formula

    [Rh(COD).sub.2 ].sup.+ BF.sub.4 --                         VII

in which COD stands for 1,5-cyclooctadiene.

The catalysts of the formula I according to the invention aredistinguished by a high hydrogenation activity and enantioselectivityduring hydrogenation both in water and in organic solvents.

For the abovementioned reasons, they are particularly suitable forasymmetric hydrogenation of IV in an aqueous or water-containing medium.When the (R,R)-3,4-bis(diarylphosphino)pyrrolidine-Rh complex is used,the L-acyl- or L-alkoxy (L-aryloxy)-Ptc is formed in high opticalyields.

The hydrogenation is advantageously carried out in water or a mixture ofwater/alcohol (for example methanol or ethanol). The substrateconcentration can extend from a 0.01 molar solution to a solutionsaturated in substrate. The hydrogen pressure can be between normalpressure and about 80 bar, preferable between 20 and 50 bar, and thereaction temperature can be between 0° and +70° C., preferably between30° and 50° C. The compounds of the formula I and IV are advantageouslyemployed in amounts such that the molar ratio of substrate to catalystis as high as possible, and it can be up to 30,000:1. The optimum liesat a ratio of about 10-15,000:1. In contrast, if methanol is used as thesolvent, molar ratios of not more than 3,000:1 can be achieved

The catalysts according to the invention can also be successfully usedin an alcoholic phase, but there are in general no advantages here overthe catalysts known from DE-OS No. 3,609,818. The same applies to theiruse for the asymmetric hydrogenation of other unsaturated acylated aminoacids of the α-aminoacrylic acid type ##STR6## Because of thesensitivity of the optically active 3,4-bis(diarylphosphino)pyrrolidinesand the rhodium complexes according to the invention containing them aschiral ligands towards oxygen, it is advantageous for all the reactionsto be carried out under an inert gas atmosphere, for example undernitrogen or argon, and also for the reaction products to be kept underan inert gas. It is moreover advisable also to carry out thehydrogenations under anaerobic conditions.

The following examples are intended to illustrate the invention in moredetail, without a limitation thereby being intended.

EXAMPLE 1 Preparation of (3R,4R)-3,4-bis(di-p-tolylphosphino)pyrrolidine

33.15 g (0.155 mol) of di-p-tolylphosphane and 3.56 g (0.155 mol) ofsodium are heated under reflux in 150 ml of tetrahydrofuran until allthe sodium has dissolved (about 6 hours). The tetrahydrofuran isstripped off, the residue is taken up in 160 ml of anhydrousdimethylformamide, the mixture is cooled to -20° C. and 13.6 g (40 mmol)of (3S,4S)-3,4-bis(methanesulfonyl)pyrrolidinium bromide (preparedaccording to EP-OS No. 151,282) are added all at once. The mixture isstirred at this temperature for a further hour and is then placed in therefrigerator overnight. The solvent is stripped off in vacuo, the deepred residue is partitioned between 140 ml of water and 140 ml of diethylether, the aqueous phase is extracted once again with 70 ml of diethylether, 160 ml of 1 N HCl solution are added to the combined ether phasesand the mixture is stirred for 3 hours. The solid which has precipitatedis filtered off with suction, rinsed with 50 ml of water and 100 ml ofdiethyl ether and partitioned between 150 ml of toluene and 60 ml of 1 Nsodium hydroxide solution. The organic phase is dried over Na₂ SO₄ andconcentrated to about 20 ml and the product is precipitated with 150 mlof hexane.

Yield: 10.9 g (55% of theory).

Melting point: 74°-76° C.

[α]_(D) ^(RT) =+106° (c=0.7; toluene)

³¹ P-NMR (CDCl₃)δ(ppm): -8.2, s

EXAMPLE 2 Preparation of the polyethylene glycol (molecular weight10,000) derivative of[((3R,4R)-3,4-bis-(diphenylphosphphino)pyrrolidine) (COD)Rh]BF₄

10 g (1 mmol) of polyethylene glycol (molecular weight 10,000) aredissolved in 260 g of dioxane in a dried 500 ml flask. 60 g of dioxaneare distilled off in order to remove traces of water from thepolyethylene glycol. After cooing to room temperature, 0.37 g (2.1 mmol)of 2,4-toluylene diisocyanate are added and the mixture is then stirredovernight. 42 g of the solution are removed for titration forisocyanate. A content of 1.36 mmol of isocyanate results for theremaining solution. 0.53 g (1.2 mmol) of(3R,4R)-3,4-bis-(diphenylphosphino)pyrrolidine (prepared according toEP-OS No. 151,282) is added, the mixture is stirred for 2 hours and then15 ml of methanol and after a further 15 minutes 0.41 g (1 mmol) of[Rh(COD)₂ a BF₄ are added. After stirring overnight, the mixture isconcentrated to about 40 ml in vacuo and the product is precipitated byaddition of 200 ml of hexane. 9.1 g (99% of theory) of the pale yellowproduct of melting point 52°-55° C. are obtained by filtration withsuction and drying under a high vacuum.

Average molecular weight, based on rhodium: 9,100

EXAMPLE 3 Preparation of the polyethylene glycol (molecular weight10,000) derivative of[((3R,4R)-3,4-bis-(di-p-tolylphosphino)pyrrolidine) (COD)Rh]BF₄

10 g (1 mmol) of polyethylene glycol (molecular weight 10,000) and 0.37g (2.1 mmol) of 2,4-toluylene diisocyanate are reacted analogously toExample 2. After stirring overnight, 20% of the batch is removed fortitration for isocyanate. A content of 1.52 mmol of isocyanate resultsfor the remaining solution. 0.65 (1.3 mmol) of the(3R,4R)-3,4-bis(di-p-tolylphosphino)pyrrolidine prepared according toExample 1 are added, the mixture is stirred for 2 hours and then 15 mlof methanol and after a further 15 minutes 0.45 g (1.1 mmol) of[Rh(COD)₂ ]BF₄ are added. After stirring overnight, the mixture isconcentrated to about 40 ml in vacuo and the product is precipitated byaddition of 200 ml of hexane. 9.3 g (100% of theory) of the pale yellowproduct of melting point 53°-55° C. are obtained after filtration withsuction and drying under a high vacuum.

Average molecular weight, based on rhodium: 8,400.

EXAMPLE 4 Preparation of the polyethylene glycol monomethyl ether(molecular weight 5,000) derivative of[((3R,4R)-3,4-bis-(tolylphosphino)pyrrolidine)(COD)Rh]BF₄

10 g (2 mmol) of polyethylene glycol monomethyl ether (molecular weight5,000) and 0.37 g (2.1 mmol) of 2,4-toluylene diisocyanate are reactedanalogously to Example 2. After stirring overnight, 20% of the batch areremoved for titration for isocyanate. A content of 1.4 mmol ofisocyanate results for the remaining solution. 0.59 g (1.2 mmol) of(3R,4R)-3,4-bis(di-p-tolylphosphino)pyrrolidine prepared according toExample 1 is added, the mixture is stirred for 2 hours and then 15 ml ofmethanol and after a further 15 minutes 0.41 g (1 mmol) of [Rh(COD)₂]BF₄ are added. After stirring overnight, the mixture is concentrated toabout 40 ml in vacuo and the product is precipitated by addition of 200ml of hexane. 8.9 g (97% of theory) of the pale yellow product ofmelting point 55.5°-58° C. are obtained after filtration with suctionand drying under a high vacuum.

Average molecular weight, based on rhodium: 9,200.

EXAMPLE 5 Preparation of the polyethylene glycol (molecular weight 350)monomethyl ether derivative of[((3R,4R)-3,4-bis-(diphenylphosphino)pyrrolidine)(COD)Rh]BF₄

1.05 g (3 mmol) of polyethylene glycol (molecular weight 350) monomethylether are dried at 40° C. under a high vacuum for 2 hours and then takenup in 40 ml of absolute dioxane, and 10 ml of dioxane are distilled offin order to remove the last traces of water. 0.52 g (3 mmol) of2,4-toluylene diisocyanate are added and the mixture is stirredovernight. 20% of the batch is removed for titration for isocyanate. Acontent of 2 mmol of isocyanate results for the remaining solution. 0.79g (1.8 mmol) of (3R,4R)-3,4-bis-(diphenylphosphino)pyrrolidine (preparedaccording to EP-OS No. 151,282) is added, the mixture is stirred for 2hours and then 5 ml of methanol and after a further 15 minutes 0.67 g(1.65 mmol) of [Rh(COD)₂ ]BF₄ are added. After stirring overnight, thesolvent is stripped off and the residue is dried under a high vacuum.

Yield: 2.49 g (98% of theory) of pale yellow melting point 81°-85° C.

Average molecular weight, based on rhodium: 1.550.

EXAMPLE 6 Hydrogenations with the rhodium complex prepared according toExample 2

66 mg of the rhodium complex are dissolved under inert gas in a degassedsolution of 17.6 g of N-acetyl-2,3-dehydrophosphinothricin (Δ-Ac-Ptc,prepared according to DE-OS No. 3,609,818) in 50 ml of water. Thissolution is introduced, in counter-current with N₂, into a 200 mlstainless steel autoclave which has a glass insert and magnetic stirrerand has first been freed from oxygen by means of N₂.

After flushing with H₂, H₂ is forced in up to a pressure of 50 bar, theautoclave is heated to 50° C. and the stirrer is started. After 18hours, the H₂ uptake has ended and the pressure has fallen to 34 bar.The autoclave is let down and emptied. A 6 N hydrochloric acid solutionis prepared by addition of an equivalent amount of concentratedhydrochloric acid to the reaction solution and is heated under refluxfor 8 hours. The solution is concentrated and the residue is boiled upwith 40 ml of ethanol, in order to dissolve the catalyst constituents,and then filtered off with suction, rinsed with 15 ml of ethanol anddried.

Conversion: 100%.

Yield of L-phosphinothricin hydrochloride: 16.5 g (95.3% of theory).Melting point: 194°-197° C. (decomposition).

[α]_(D) ²² =+23.2° (c=1; 1 N HCl).

This corresponds to an optical yield of 89.9% based on [α]_(D) ²²=+25.8° (c=1; 1 N HCl) for optically pure L-phosphinothricinhydrochloride (DE-OS No. 3,609,818).

(b) Comparison

Analogously to Example 6a, 73 mg of the rhodium complex are dissolved in50 ml of a methanolic solution saturated with 4.4 g of Δ-Ac-Ptc and themixture is hydrogenated at 30° C. under an initial H₂ pressure of 30bar. After 2 hours, the H₂ uptake has ended. The autoclave is let down,the reaction solution is concentrated and the residue is taken up in 6 Nhydrochloric acid. Further working up is carried out analogously toExample 6a. Conversion 100%.

Yield of L-phosphinothricin hydrochloride: 4.0 g (92.4% of theory);[α]_(D) ²² =+23.3° (c=1; 1 N HCl) corresponding to an optical yield of90.3%.

If the amounts used are 8.8 g of Δ-Ac-Ptc and 73 mg of the rhodiumcomplex in 50 ml of water under an initial H₂ pressure of 50 bar at 30°C., the hydrogenation has ended after 8 hours. 8.1 g (93.5% of theory)of L-phosphinothricin hydrochloride with an optical rotation [α]_(D) ²²of +23.0° (c=1; 1 N HCl), corresponding to an optical yield of 89.1%,are obtained.

EXAMPLE 7 Hydrogenations with the rhodium complex prepared according toExample 3

8.8 g of Δ-Ac-Ptc and 67 mg of the rhodium complex are hydrogenated in50 ml of water at 30° C. under an initial H₂ pressure of 50 baranalogously to Example 6. After 8 hours, the H₂ uptake has ended. 8.0 g(92.4% of theory) of L-phosphinothricin hydrochloride with an opticalrotation [α]_(D) ²² of +22.4° (c=1; 1 N HCl), corresponding to anoptical yield of 86.8%, are obtained.

(b) Comparison

The hydrogenation of a saturated solution of 4.4 g of Δ-Ac-Ptc and 67 mgof the rhodium complex in 50 ml of methanol at 30° C. under an initialH₂ pressure of 35 bar gives, after 1.5 hours, 3.9 g (90.1% of theory) ofL-phosphinothricin hydrochloride with an optical rotation of [α]_(D) ²²=+23.2° (c=1; 1 N HCl), corresponding to an optical yield of 89.9%.

EXAMPLE 8 Hydrogenations with the rhodium complex prepared according toExample 4

8.8 g of Δ-Ac-Ptc and 74 mg of the rhodium complex are hydrogenated in50 ml of water at 30° C. under an initial H₂ pressure of 35 baranalogously to Example 6. After 3 hours, the H₂ uptake has ended. 4.0 g(92.4% of theory) of L-phosphinothricin hydrochloride with an opticalrotation of [α]_(D) ²² =_(D) +22.3° (C=1; 1 N HCl) corresponding to anoptical yield of 86.4%, are obtained.

(b) Comparison

Hydrogenation of a saturated solution of 4.4 g of Δ-Ac-Ptc in 50 ml ofmethanol with the addition of 74 mg of the rhodium complex at 30° C.under an initial H₂ pressure of 35 bar gives, after 1.5 hours, 3.9 g(90.1% of theory) of L-phosphinothricin hydrochloride with an opticalrotation of [α]_(D) ^(RT) of +23.7° (c=1; 1 N HCl), corresponding to anoptical yield of 91.9%.

What is claimed is:
 1. A rhodium complex of the formula

    [A--O--(CH.sub.2 CH.sub.2 O).sub.n --A].sup.2+ 2 X.sup.-   Ia

and

    [A--O--(CH.sub.2 CH.sub.2 O).sub.m --CH.sub.3 ].sup.+ X.sup.-Ib

in which n=5-1,000, m=5-250, X⁻ a tetrafluoroborate, hexafluorophosphateor per-chlorate anion, A is a radical of the formula ##STR7## (en)₂represents two molecules of a monoolefin or one molecule of a diolefinAr is phenyl or phenyl which is substituted by one or two alkyl groupshaving 1 or 2 carbon atoms and R¹ is an arylene or alkylene bridge.
 2. Arhodium complex of the formula Ia as claimed in claim 1, in which n is50 to 250, corresponding to an average molecular weight of 2,000 to11,000.
 3. A rhodium complex of the formula Ib as claimed in claim 1, inwhich m is 50 to 150, corresponding to an average molecular weight of2,000 to 6,000.
 4. A rhodium complex as claimed in claim 1,in which R¹is the divalent alkylene or arylene radical of a diisocyanate from thegroup comprising hexamethylene diisocyanate, isophorone diisocyanate,diphenylmethane diisocyanate and 2,4-toluylene diisocyanate.
 5. Arhodium complex as claimed in claim 4, in which R¹ is 2,4-toluylenediisocyanate.
 6. A process for the preparation of a rhodium complex ofthe formula Ia or Ib as defined in claim 1, which comprises reacting apolyglycol of the formula HO--(CH₂ CH₂ O)_(n) --H or a polyglycolmonomethyl ether of the formula HO--(CH₂ CH₂ O)_(m) --CH₃ with adiisocyanate of the formula R₁ (N═C═O)₂, derivatizing the resultingaddition product with a 3,4-bis(diarylphosphino)pyrrolidine of theformula ##STR8## and reacting the reaction product with a rhodiumcomplex of the formula

    [Rh(en).sub.2 Y].sub.2                                     VI

(Y=Cl, Br or I) and an alkali metal salt or silver salt oftetrafluoroboric acid, hexafluorophosphoric acid or perchloric acid. 7.A process for the preparation of a rhodium complex of the formula Ia orIb as defined in claim 1, in which (en)₂ is 1,5-cyclooctadiene, and X⁻is tetrafluoroborate, which comprises reacting a polyglycol of theformula HO--(CH₂ CH₂ O)_(n) --H or a polyglycol monomethyl ether of theformula HO--(CH₂ CH₂ O)_(m) --CH₃ with a diisocyanate of the formula R¹(N═C═O)₂, derivatizing the resulting addition product with a3,4-bis(diarylphosphino)pyrrolidine of the formula ##STR9## and reactingthe reaction product with a rhodium complex of the formula

    [Rh(COD).sub.2 ].sup.+ BF.sub.4 -

in which COD denotes 1,5-cyclooctadiene.